Electrochemical conversion cells, commonly referred to as fuel cells, which produce electrical energy by processing first and second reactants, e.g., through oxidation and reduction of hydrogen and oxygen. By way of illustration and not limitation, a typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane) that is positioned between a pair of gas diffusion media layers and catalyst layers. A cathode plate and an anode plate are positioned at the outermost sides adjacent the gas diffusion media layers, and the preceding components are tightly compressed to form the cell unit.
The voltage provided by a single cell unit is typically too small for useful applications. Accordingly, a plurality of cells are typically arranged and connected consecutively in a “stack” to increase the electrical output of the electrochemical conversion assembly or fuel cell.
The catalyst layers can be made of nanostructured thin support materials. The nanostructured thin support materials have particles or thin films of catalyst on them. The nanostructure thin catalytic layers can be made using well known methods. One example is nanostructured thin film (NSTF) catalyst layers available from 3M. The nanostructured thin catalytic layers can be transferred directly to a proton exchange membrane, such as a Nafion® membrane, using a hot press lamination process, for example. The polyimide substrate is then peeled off, leaving the layer of whiskers attached to the membrane.
These types of nanostructured thin catalytic layers have demonstrated high catalytic activities, which is helpful to reduce the platinum utilization in fuel cell stacks. Most importantly, because the supporting layer is not made of carbon as in the traditional platinum catalysts for fuel cell application, the nanostructured thin catalytic layers are more resistant to corrosion under certain fuel cell operating conditions, and thus improve the fuel cell's durability.
However, an MEA made with this type of whisker catalyst layer is very sensitive to water management and has a narrow range of operating conditions (i.e., it cannot be too dry or too wet) to provide good performance. If the fuel cell is operated under wet conditions, the thin layer of whiskers, which is less than 1 μm thick, cannot provide enough storage capacity for the product water, resulting in flooding. Under dry conditions, it is believed that not all portions of the whiskers are utilized to catalyze the reaction due to poor proton transfer characteristics.
Therefore, there is a need for a method of operating a fuel cell which can provide good performance over a wider range of operating conditions.